One of them is described in U.S. Pat. No. 5,446,702. It comprises, in a watch, an electronic circuit for assessing the charge of its battery, a device for displaying the date and an independent motor to drive this display device. When the charge falls below a limit value, the electronic circuit gives the order to the motor to drive the display device to a position indicating that the battery is nearly discharged. The information thus given is therefore of the binary type. The person wearing the watch thus has a few days to change its battery.
Another device of this type is described in document JP 58061488. This concerns more specifically a watch including means for converting the battery charge into an operating time. This watch comprises a counter and a display. The counter is regularly incremented and its content, which corresponds to the remaining lifetime, is indicated by means of the display. The wearer thus knows the battery's life expectancy at any time. For such a device to be reliable, a large safety margin must be adopted. This is because the capacity of a battery cannot be measured precisely without discharging it. This capacity varies considerably from one battery to another, notably due to storage conditions. Accordingly, there is no doubt that, in many cases, the battery will be changed when its power reserve could still ensure operation for several weeks, or even several months. If the safety margin is inadequate, it is likewise possible that the reserve may be smaller than that assessed, so that the watch stops before the indicator is at zero, which is even more troublesome.